1. Field of the Invention
The present invention is directed to a polarization independent liquid crystal display using reflective spatial light modulators, and more particularly, to a display having reflective spatial light modulators with liquid crystal phase gratings that exhibit polarization independent diffraction of incident light.
2. Discussion of the Prior Art
Increasingly, cathode ray tube (CRT) displays are being replaced with liquid crystal displays (LCDs). LCDs use spatial light modulators (SLMs) to form images. One type of an LCD is an active-matrix-driven liquid crystal display (AMLCD) that uses SLMs. The SLMs may be either transmissive or reflective.
FIG. 1 shows a basic structure of a display 100 having conventional active-matrix-driven liquid crystal transmissive spatial light modulators (AM LC SLMs). The display 100 has a back polarizer 110 and a front polarizer 115 which faces a viewer. Front and back glass substrates 120, 125 are located between, and next to, the back and front polarizers 110, 115, respectively. The AM LC SLMs, whether transmissive or reflective, typically have repetitive unit cells or picture elements (pixels) 130. FIG. 1 shows 9 pixels in a 3.times.3 pixel array.
An array of transistors 135, such as field effect transistors (FETs), is formed on the back glass substrates 125. Each FET 135 in the array has its own transparent conductive electrode 140, referred to as a pixel electrode 140. Each FET 135 and pixel electrode 140 are part of one pixel (or subpixel) of an image formed on a screen (not shown) located between a viewer (not shown) and the front polarizer 115 of the display 100. Collectively, the pixel electrodes 140 form an array of pixel electrodes.
On the front substrate 120, a transparent conductive front electrode 145 is formed, which is common to all the FETs 135 in the transistor array. The front electrode 145 is referred to as a counter-electrode. Optional color filters 150 arranged in an array may be formed between the front substrate 120 and the common counter-electrode 145. The color filters include red, green and blue filters, where each pixel has its own filter.
A liquid crystal (LC) medium 155 is sandwiched between the transparent conductive pixel and common counter-electrodes 140, 145. Two alignment layers, i.e., front and back alignment layers (not shown), such as rubbed polyimide films, are formed between the LC medium 155 and the pixel and common electrodes 140, 145, respectively. A back-light source 160 illuminates the back of the display panel 100, where light rays 165 from the back-light source 160 are incident on the back polarizer 110.
Each FET 135 is an on/off transistor switch that supplies a voltage to the pixel electrode 140 in the ON condition. This in turn generates an electric field between the pixel electrode 140 and the common counter-electrode 145. The electric field aligns molecules of the LC medium 155. This alignment causes light passing through the LC medium 155, from the back-light source 160, to form an image a screen (not shown) located between the front polarizer 115 and a viewer (not shown).
Instead of transmissive SLMs, where the pixel electrodes 140 are transparent, reflective SLMs may be used having reflective pixel electrodes. For reflective AM LC SLMs having reflective pixel electrodes, the transparent conductive pixel electrode 140 is replaced with a reflective metal electrode. Each metal pixel electrode of a reflective SLM typically occupies a larger area than a corresponding transparent pixel electrode 140 of a transmissive SLM.
The additional area of the reflective metal pixel electrode covers the FET 135.
For reflective AM LC SLMs, there is no need for the back-light source 160 used with transmissive SLMs. Instead, ambient light or another light source illuminates the display panel from the front of the panel, e.g., from the top or front polarizer 115 shown in FIG. 1. A more detailed description of a display having reflective AM LC SLMs is given in connection with FIG. 8.
FIG. 2 shows an equivalent circuit 200 of one of the pixels 130 shown in FIG. 1. Although FIG. 1 is a display using transmissive SLMs, FIG. 2 is an equivalent circuit 200 for both transmissive and reflective SLMs. A display using reflective SLMs is shown in FIGS. 3 and 4.
As shown in FIG. 2, the gate 205 of the FET 135 is connected to a gate bus line 210, while the FET drain 215 is connected to a data bus line 220. The source 225 of the FET 135 is connected to the pixel electrode 140, which is shown in FIG. 1 as the transparent pixel electrode, and is also shown in FIGS. 3, 4 as a reflective pixel electrode 140'. The LC medium 155 of FIG. 1 is equivalent to a capacitor 230, which has one terminal connected to the pixel electrode 140 and another terminal connected to the common transparent counter-electrode 145.
A storage capacitor 240 provides parallel capacitance to the LC capacitor 230. The storage capacitor 240 is terminated on a common line 260, which is common to all the storage capacitors 240 in the display. Another alternate design for a storage capacitor is to replace the storage capacitor 240 with a storage capacitor 250, which is connected from the pixel electrode 140 to the gate bus line 210.
When a voltage below a threshold voltage is applied on the gate bus line 210, the FET 135 is in an OFF condition (OFF state). The OFF FET 135 acts as an open switch and separates the data bus line 220 from the pixel electrode 140. This isolates the potentials on data bus line 220 and the pixel electrode 140 from each other.
When a voltage larger than the threshold voltage is applied to the gate bus line 210, the FET 135 is turned ON (ON state) and has a low impedance between its source 225 and drain 215. The ON FET 135 acts as a closed switch and connects the data bus line 220 to the pixel electrode 140. This transfers the data voltage on the data bus line 220 to the pixel electrode 140.
In the ON state, varying the data voltage on the data bus line 220 varies the voltage applied to the pixel electrode 140. The different voltages applied to the pixel electrode 140 variably turn on the liquid crystal cell 230. Varying the pixel voltage (on the pixel electrode 140) varies the intensity of light as it passes through the liquid crystal cell 155 shown in FIGS. 1, 3, 4, and represented as the LC capacitor 230 in FIG. 2. This results in displaying different scales of gray color on a screen (not shown) located between a viewer (not shown) and the front polarizer 115 shown in FIG. 1.
FIGS. 3 and 4 show cross sectional and perspective views of a conventional reflective display 300 using an array of reflective liquid crystal spatial light modulators (LC SLMs). The array of FETS 135 are formed on the substrate 125, which is a silicon (Si) wafer, for example. Each FET 135 drives one of the reflective SLMs in the SLM array as described below.
FIG. 3 is a cross sectional view of a single reflective liquid crystal light valve or SLM of the conventional reflection liquid crystal (LC) display 300. The FET 135 is formed between field oxide regions 305 on the semiconductor Si substrate 125. The field oxide regions 305 separate the FET 135 from other FETs or devices formed on the substrate 125. The FET 135 has source and drain regions 225, 215 which are formed in the substrate 125. The source and drain regions 225, 215 are separated by a channel region 310.
Over the channel region 310, a gate insulating film 315 is formed. Illustratively, the gate insulating film 315 is an SiO.sub.2 layer having a thickness which is approximately from 150 to 500 angstroms (.ANG.). A polysilicon gate electrode 205, e.g., having a thickness of approximately 0.44 micron (.mu.), is formed over the gate insulating film 315.
A layer of dielectric or insulator material, such as an SiO.sub.2 layer 320, is formed over the FET 135 and field oxide regions 310. The storage capacity line 260, also shown in FIG. 2, is formed over a portion of the SiO.sub.2 layer 320 so that the storage capacity line 260 extends over portions of the source 225 and the field oxide regions 305 adjacent thereto. A second SiO.sub.2 layer 325 is formed over the storage capacity line 260 and exposed portions of the first SiO.sub.2 layer 320. The two SiO.sub.2 layers 320, 325 act as inter-layer insulating films.
First and second via holes are formed extending through both SiO.sub.2 layers 320, 325 to expose portions of the source and drain regions 225, 215, respectively. A conductive source line 330 and the conductive data bus line 220, which is also shown in FIG. 2, are formed in the first and second via holes, respectively. The conductive source and data lines 330, 220 extend over portions of the second SiO.sub.2 layer 325 and are electrically connected to the source and drain regions 225, 215, respectively. Illustratively, the source and data lines 330, 220 are aluminum (Al) and have a thickness 335 of approximately 0.7 microns.
A third silicon oxide SiO.sub.2 film 340, acting as an inter-layer insulating film, is formed over the source and data lines 330, 220 and exposed portions of the second silicon oxide SiO.sub.2 layer 325. Over the third oxide Sio.sub.2 layer 340, an optical absorbing layer 345 is formed. The optical absorbing layer 345, which has a thickness of approximately 160 nano-meters (nm), is formed of three layers that are laminated over each other in the following order: A titanium (Ti) layer having a thickness of approximately 100 .ANG.; an aluminum (Al) layer having a thickness of approximately 1000 .ANG.; and a titanium nitride (TiN) layer having a thickness of approximately 500 .ANG..
Laminating these three layers so as to form the optical absorbing layer 345 with a thickness of approximately 160 nm, reduces reflection of light, e.g., having a wavelength from 345 to 700 .ANG., that enters the optical absorbing layer 345 to result in a reflection factor of approximately 25%. The light that enters the optical absorbing layer 345 is shown as arrow 350 in FIG. 3.
In addition, the optical absorbing layer 345 prevents the light 350 from being transmitted to the FET 135 to result in a transmission factor of approximately 0%. The optical absorbing layer 345 improves contrast of images and prevents leakage currents in the FET 135.
A silicon nitride film 355, having a thickness of approximately from 400 to 500 nm, is formed on the optical absorbing layer 345. Next, an Al light reflecting film 140' having a thickness of approximately 150 nm, also shown in FIG. 2 as reference numeral 140 and referred to as the pixel electrode, is formed over the silicon nitride film 355.
A via hole is formed to expose a portion of the source line or electrode 330 of the FET 135. The via hole penetrates through the light reflecting film or pixel electrode 140', the silicon nitride film 355, the optical absorbing layer 345, and the third silicon oxide SiO.sub.2 film 340.
A conducting stud, 360, such as a tungsten (W) stud is formed in the via by a chemical vapor deposition CVD method, for example. The tungsten stud 360 electrically connects the source line or electrode 330 to the light reflecting film or pixel electrode 140'. To prevent electrical connection to the tungsten stud 360, the optical absorbing layer 345 is removed from around the tungsten stud 360.
As more clearly shown in the perspective view of the display 300 in FIG. 4, the light reflecting film or pixel electrode 140' is separated from adjacent pixel electrode 140'. Illustratively, the reflective pixel electrode 140' are spaced apart from each other at a specified interval of about 0.5 to 1.7 microns. Each reflective pixel electrode 140', along with its associated FET 135, form a subpixel. For example, three subpixels for red, green and blue components of light form a pixel.
At selected locations of the array of subpixels, pillar-shaped spacers 365 are formed in the space that separates the reflective pixel electrodes 140' from each other. Illustratively, the pillar-shaped spacers 365 are SiO.sub.2 spacers having a width 370 of approximately 1 to 5 microns. The height 375 of each spacer 365 is determined according to the desired cell gap, which is filled with the liquid crystal (LC) 155. The spacers 365 are provided throughout the substrate at specified intervals in order to retain the desired cell gap or thickness d of the LC material 155.
Note, the width 370 of each spacer 365, which is about 1-5.mu., is the same order as the distance of about 0.5-1.7.mu. that separates the reflective pixel electrode 140'. This provides minimum overlap of the spacers 365 with the reflective pixel electrode 140', which in turn minimizes any reduction of the numerical aperture of each subpixel resulting from the pillar-shaped spacer 365.
The counter-electrode 145, which is formed on the glass protect substrate 120, is attached over the spacers 365. The counter-electrode 145 and glass substrate 120 are also shown in FIG. 1. The glass protect substrate 120 is the front portion of the display 300, i.e., the portion facing a viewer. As described in connection with FIG. 1, the counter-electrode 145 is transparent and common to all the pixels. Illustratively, the counter-electrode 145 is an indium titanium oxide (ITO) transparent electrode.
Attaching the counter-electrode 145 over the pillar-shaped spacers 365 forms the cell gap. The liquid crystal (LC) layer 155, in which a liquid crystal material is sealed, is formed in the cell gap between the light reflecting film or pixel electrode 140' and the counter-electrode 145. Orienting films (not shown) are also formed over the pixel electrode 140' and the counter-electrode 145 to orient the liquid crystal molecules.
Illustratively, as shown in FIG. 4, each pixel electrode 140', which defines a subpixel, has a square shape with a side of approximately 17 microns. To form the display 300, the subpixels are arranged in a matrix or array of 1280 rows and 1600 columns, for example.
In the reflective liquid crystal light valve or SLM, comprising the LC material 155 sandwiched between the common transparent ITO counter-electrode 145 and the reflective pixel electrode 140', light 350 entering from the glass protect substrate 120 reflects from the reflective pixel electrode 140'. The pixel electrode 140' also functions as a display electrode for applying a voltage to the liquid crystal layer 155. The FET 135 functions as a switching element for providing a signal voltage from the data line 220 to the pixel electrode 140', when a control voltage on the gate 205 turns on the FET 135, as described in connection with FIG. 2.
An image is projected, from the front glass substrate 120, onto a screen (not shown) located between a viewer (not shown) and the front glass substrate 120, when the light 350 that enters the front glass protect substrate 120 travels through LC material 155 and reflects back to the front glass substrate 120. This light is reflected from the reflective pixel electrode 140'. Depending on the voltage of the pixel electrode 140', which voltage affects alignment of the LC material 155, the light reflected from the reflective pixel electrode 140' either exits the front glass protect substrate 120, e.g., to form an image on the screen, or is blocked by an analyzer (not shown) from reaching the screen.
The light polarization rotating properties of the LC material 155 results from varying the direction of the liquid crystal molecules (not shown) in accordance to a voltage applied between the reflective pixel electrode 140' and the transparent ITO common counter-electrode 145. As described in connection with FIG. 2, this voltage is supplied from the data bus line 220 to the pixel electrode 140' when the FET 135 is turned on in response to a control signal on the gate bus line 210 (FIG. 2), which is connected to the gate 205 of the FET 135.
Depending on the voltage applied to the pixel electrode 140', the directors' orientation of the LC material 155 changes. This varies the state of the polarization of light that is incident on the pixel electrode 140', that reflects therefrom, and that exits the front glass protect substrate 120. Based on this variation of the state of light polarization, an image is formed on a screen located between the front glass protect substrate 120 and a viewer, as further described in connection with FIG. 8.
According to the prior art, the LC medium 155 used in reflective spatial light modulators (SLMs) requires either a linearly-polarized or randomly polarized incident light 350. The requirement of a polarized incident light results in a poor optical through-put, because more than half of the incident light is absorbed or rejected by the polarizer 110 (FIG. 1), which is located over the front glass protect substrate 120. That is, only one of the vertical or horizontal polarizations of light is used thus limiting the display efficiency to a theoretical maximum of 50%.
To increase the display or SLM efficiency, both vertical or horizontal polarizations of light are used. In this case, the LC medium 155 has been used in polarization independent LC devices based on scattering or diffraction. This allows the entire input intensity of both orthogonal horizontal and vertical polarizations to be used.
One typical example of scattering LC SLMs or light valves used in displays is referred to as a polymer-dispersed liquid crystal. However, polymer-dispersed liquid crystal light valves cannot operated below 4 volts (V). In addition, the polymer-dispersed liquid crystal has a large hysteresis as seen from plots of transmission or reflection versus voltage. Thus, a full-color display cannot be realized by using polymer-dispersed liquid crystal.
Instead of scattering LC SLMs, the other approach to achieve a polarization independent light valve having LC material uses light diffraction effect of LC phase gratings.
Diffractive light valves using LC material have been previously proposed. These proposed prior art SLMs use nematic LC mixtures with positive dielectric anisotropy. In addition, the LC molecules adjacent to the cell substrates are in homogeneous, i.e., parallel, alignment.
One such proposal of an LC grating design, based on field-induced tunable birefringence, is described in Y. Hori, K. Asai, "Field Controllable Liquid-Crystal Phase Grating", IEEE Trans. Elec. Dev. Vol. 26, pp. 1734-1737 (1979), hereinafter referred to as Hori. However, the Hori configuration requires the use of polarizers which lower the transmission of light.
The LC light valve proposed by Hori uses an untwisted LC material which has polarization dependent performance and requires high voltage to achieve high contrast. In addition, the Hori proposed LC light valve also requires interdigitated electrodes within each pixel, resulting in increased possibility of shorts and undesired interconnections across electrodes.
Other proposals have investigated polarization independent LC gratings. One such proposal is disclosed by M. Fritsch, H. Wohler, G. Haas, D. Mlynski, "Liquid Crystal Phase Modulator for Large Screen Projection", IEEE Trans. Elec. Dev. Vol. 36, pp. 1882-1887 (1989), hereinafter referred to as Fritsch. Fritsch proposed an LC light valve using a 90.degree. and a 180.degree. twisted LC material. However, the Fritsch light valve demonstrated only about a 0.5 .pi. (i.e., 90.degree.) phase difference between ON and OFF states. The result is a reflective light valve based on field-controlled birefringence difference between alternating strips inside each pixel.
All the above approaches have the further disadvantage that electrodes are required to be patterned with high resolution.
To avoid the use of the interdigitated electrodes, the use of pattern alignment to generate phase gratings has been suggested in W. Gibbons, P. Shannon, S. Sun, B. Swetlin, "Surface-Mediated Alignment of Nematic Liquid Crystals with Polarized Laser Light", Nature Vol. 351, pp. 49-501991), hereinafter referred to as Gibbons. Because the differently patterned domains of the LC material requires no separation, diffraction efficiency is increased and the risk of shorts between electrodes is reduced.
Other proposals have demonstrated the use of pattern alignment with an optically active device for transmissive SLMs. One such proposal is disclosed in P. Bos, J. Chen, J. Doane, "An Optical Active Diffractive Device for a High Efficient Light Valve", SID 95 DIGEST, pp. 601-604 (1995), hereinafter referred to as Bos. Bos proposed several schemes. The first scheme is an LC diffractive light valve based on a two domain tunable birefringence (TD-TBD) light valve.
FIG. 5 shows a top view of the LC material of one pixel of the Bos TD-TBD light valve 400. The Bos TD-TBD light valve has alternating strips 410, 420 of tunable birefringence LC material. The projections of the LC material's optical axes or directors 430 of one strip 410 are orthogonal to the directors 430 of adjacent strips 420. The alternating strips 410, 420 are formed by patterning the alignment layer of the LC medium, which alignment or orienting layer is formed on the pixel electrode 140, for example.
The TD-TBD LC phase gratings that have alternating strips, which are also referred to as Freedericksz domains, are "No-Twist" gratings. Within each Freedericksz domain, the LC directors are aligned uniformly across the cell substrates. For example, in the strip referenced by numeral 410 in FIG. 5, all the LC directors 430 are vertical, while all the LC directors 430 in the strip referenced by numeral 420, which is adjacent to the first strip 410, are horizontal.
When the devices have a voltage applied such that the effective value of d.DELTA.n/.lambda. is a multiple of a 1/2, then all polarizations of light is diffracted. However, if d.DELTA.n/.lambda. is 0 or a multiple of 1, then no diffraction occurs. Note that d, which is shown in FIG. 3, is the cell gap or the thickness of the LC material 155; .lambda. is the wavelength of incident light 350; and .DELTA.n is optical refractive index anisotropy or birefringence of the LC material 155. In particular, .DELTA.n is the difference in the refractive indices of the LC material 155 for extraordinary light n.sub.e (e.g., horizontal polarization) and ordinary light n.sub.o (e.g., vertical polarization) ; .DELTA.n=n.sub.e -n.sub.o. When .DELTA.n is positive, then the LC material is referred to as having positive birefringence. Similarly, when .DELTA.n is negative, then the LC material is referred to as having negative birefringence.
A problem with the Bos TD-TBD no-twist grating light valve is that when the LC material has positive dielectric anisotropy and the device is switched between 0 and 1/2 wave, the voltage required to get to zero retardation is undesirably high, such as above 5 volts.
The second light valve described by Bos is a two-domain 90.degree. twisted nematic (TDTN) phase grating, with adjacent domains being twisted opposite to each other. That is, adjacent domains alternate between a positive and a negative twist angle. However, the magnitude of the twist angle of each domain, excluding the direction or sign of the twist angle, is 90.degree..
FIG. 6 shows a liquid crystal (LC) configuration 500 of LC directors 510 in two adjacent strips 520, 530 of a conceptual device described in Bos. The magnitude of the twist angle of each strip 520, 530, is 90.degree.. One strip 520 has a left handed twist, e.g, having a positive sign, for a twist angle of +90.degree. shown as arrow 540, arid the other strip 530 has a right handed twist, e.g, having a negative sign, for a twist angle of -90.degree. shown as arrow 550. A reference axis 560 along the cell gap thickness d is shown in FIG. 6.
FIG. 7 shows light waves 610, 620 propagating through the two strips 520, 530 of the conceptual device 500 of FIG. 6. As shown in FIG. 7, both the polarization and the phase of the light waves 610, 620 change as they propagate through the LC of the device 500. The polarization of the incident light waves 610, 620 changes from vertical to horizontal polarization.
Because the two strips 520, 530 are twisted 90.degree., the light waves 610, 620 exit the strips 520, 530 each rotated by 90.degree.. However, since the 90.degree. twist of the two strips 520, 530 are in opposite directions, i.e., +90.degree. for one strip 520 and -90.degree. for the other strip 530, the light waves 610, 620 exit the strips 520, 530 with opposite polarizations. This results in two light waves 610, 620 being 180.degree. out of phase as they exit the strips 520, 530 of the LC conceptual device 500.
In TDTN phase gratings, if d.DELTA.n/.lambda. is chosen to satisfy the condition that d.DELTA.n/.lambda.=.sqroot.n.sup.2 -0.25, where n is an integer, then all polarizations of light passing through the LC material is diffracted.
Using TDTN phase gratings, a transmissive light valve with high contrast and relatively low-operating voltage has been demonstrated. However, while the optical diffraction efficiency for transmissive TDTN phase grating can be high, approaching 100%, nevertheless the diffraction efficiency is below 35% when used as a reflective (instead of transmissive) LC phase grating. It is desirable to lower the operating voltage of reflective LC phase gratings to around 3 volts or lower. However, a reflective TDTN LC phase grating cannot operate at 3 volts or lower.
The third light valve described by Bos is an orthogonal-twist two-domain TN (OTTDTN) having alternating TN domain strips. The alternating TN domain strips within each pixel have identical twist direction. The rub direction to align the LC medium in one domain is perpendicular to that of the other domain at the front and back surfaces of the LC cell. For example, one strip has a twist angle from 0.degree. to +90.degree., while the other strip has a twist angle from +90.degree. to +180.degree.. In comparison, in the second light valve described by Bos having the two-domain 90.degree. twisted nematic (TDTN) phase grating, one strip has a 0.degree. to +90.degree. twist angle, while the other strip has a 0.degree. to -90.degree. twist angle.
Although reflective OTTDTN LC phase gratings have a better optical diffraction efficiency than that of reflective TDTN phase gratings, the operating voltage of the OTTDTN is similar to the operating voltage of the TDTN and is unlikely to be lower than about 3.4 volts.
In using polarization-independent LC phase gratings, except for rare cases where the incident light is monochromatic and the LC cell gap is uniform with negligible deviation from a design value, it is difficult to make the non-diffracted light vanishingly small. If the non-diffracted light is collected as a signal to form the image, it is important to make the non-diffracted light vanishingly small because one of the ON or OFF states of the SLM has to be a dark state of the display. Therefore, collecting the non-diffracted light to represent a signal results in poor contrast. Accordingly, for illustrative purposes, the following description of the present invention concerns only the case of collecting diffracted light as a signal that forms an image. However, it is understood that non-diffracted light may also be collected as a signal to form an image.
Both the second and third type of light valves described by Bos, i.e., the TDTN and OTTDTN gratings, are diffractive at zero volt and less diffractive at higher voltages. Such devices are called normally-white diffractive (NWD) gratings in the case where the diffracted light is collected to represent a signal. There are other types of LC phase gratings, typically referred to as normally-black diffractive (NBD) gratings, that do not diffract light at zero volt and becomes more diffractive as the applied voltage increases.
Bos only proposed NWD LC phase gratings without any mention of the NBD LC phase gratings. The Bos proposed NWD gratings are either unsuitable for reflective SLMs or have an operating voltage which is higher than about 3 volts. That is, the Bos proposed NWD gratings cannot operate at voltages below about 3 volts.
Reflective SLMs have been used for projection displays. FIG. 8 shows a typical optical system 700 having three reflective SLMs 705, 710, 715 which use LC phase gratings to render the reflective SLMs 705, 710, 715 into polarization independent devices.
Light is emitted from a lamp 720 having a reflector 725. The emitted light passes through a relay lens 730, and is reflected by a mirrored bar (louvers) system 735 toward crossed dichroic filters 740, 745. Red, green, and blue components of the light are separated by the crossed dichroic filters 740, 745, and are directed toward the reflective SLMs 705, 710, 715, respectively. Schlerien lenses 750, 755, 760 are located between the SLMs 705, 710, 715, and the dichroic filters 740, 745, respectively. The Schlieren lenses 750, 755, 760 image each color components of the incident light onto and out of the corresponding SLMs 705, 710, 715.
For the case of NBD modes, if the LC phase grating of each reflective SLM 705, 710, 715 is not activated, the incident red, green, and blue lights are not diffracted by the LC material of the SLM 705, 710, 715. After being reflected back from the corresponding SLM 705, 710, 715, the red, green, and blue component of light are imaged by the corresponding Schlieren lenses 750, 755, 760 and are recombined by the crossed dichroic filters 740, 745. The recombined light from the dichroic filters 740, 745 are reflected by the mirrored bar system 735 back toward the relay lens 730.
Because the LC phase grating of each reflective SLM 705, 710, 715 is not activated, almost no light reflected from the SLMs 705, 710, 715 passes through the mirrored bar system 735. Thus, this light reflected from the SLMs 705, 710, 715 is not directed to a projection lens 765 located between the mirror bar 735 and a screen 770. This inactivated state of the SLM 705, 710, 715 represents the dark state of the display 700.
The bright state of the display occurs when the LC phase grating in each reflective SLM 705, 710, 715 is activated. In this bright state, light incident on the reflective SLMs 705, 710, 715 from the lamp 720 is diffracted. Most of the light reflected from the reflective SLMs 705, 710, 715 passes through the mirrored bar system 735 and is projected by the projection lens 765 onto the display screen 770 to illuminate a pixel 775 thereon.
For the NWD modes, the fully activated state of the SLMs 705, 710, 715 of FIG. 8 represents the nondiffractive state, so that the screen 770 appears dark. The fully diffractive state occurs in the quiescent state, where the SLMs are not activated, i.e., the FETs are not turned ON. In this quiescent or fully diffractive state, the screen 770 appears bright.
The present invention relates to polarization independent LC phase gratings that are fabricated into reflective SLMs e.g., for projection displays, such as the display 700 shown in FIG. 8.